This invention relates to a device for controlling the advance movement of the pattern selection drums or disks in a circular knitting machine.
Reference will be made hereinbelow exclusively to pattern drums, but it will be appreciated that the invention is also applicable in machines equipped with pattern selection devices comprising plural stacked pattern disks or wheels.
As is known, the pattern drums have a plurality of pegs or teeth arranged on the side surface thereof, in accordance with the pattern to be obtained on the final knitted product. The drums are caused to advance stepwise, thereby presenting for each knitting course a given peg or tooth arrangement, which arrangement is not required to be a different one for each course. The pegs interfere with corresponding jack selecting levers to move such levers closer to the needle cylinder (or to the lower needle cylinder, in the instance of a double cylinder machine), thereby to select, for each knitting course, those needles which are to pick up the yarn or thread from the yarn feed wherewith the drum is associated.
The advance movement of each drum must be effected at a well defined time, so that a new arrangement of pegs and levers will coincide with a fresh knitting course. For this purpose, an annular cam is made rigid with the needle cylinder which has a symmetrically walled depression at a given angular position and is followed by a square bracket lug on a support which is freely pivoted under each drum coaxially thereto. To the support, a pawl is pivoted which cooperates with a peripheral tooth formation of the drum, for advancing it. The depth of the depression in the radial direction is such as to allow, at each revolution of the cylinder, an oscillation of swing movement of the support adequate for resulting in a two-tooth advance movement of the drum, i.e. a two-step advance movement. In practice, however, the advance movement normally utilized will be a single step, and the two-step advance movement will only be performed to bring the drum back to the starting or initial conditions upon completion of one or more repeated patterns. In fact, since it is not always possible to make the width (as expressed in terms of knitting courses) of a pattern coincide with the total number of vertical rows of pegs in a drum or with a submultiple of that number (and accordingly, with the total number of advance teeth or submultiple thereof), it is at a certain moment necessary to cause the drum to advance one or more times through two-step cycles, such that at the end of the pattern (or of a finite number of repeated patterns) it has completed one revolution exactly.
In conventional techniques, the one-step advance, rather than the two-step one, is accomplished by blocking the pawl support during the return stroke at a position whereby the next advance movement, as controlled by the cam, implies for the pawl a stroke length corresponding to a single tooth of the drum, rather than two such teeth. In this case, the pawl engages the tooth directly behind the one engaged during the preceding advance movement, and the square bracket lug of the support does not follow the cam depression down to the bottom of it but remains temporarily detached therefrom, to undergo the cam effect during the second half of the depression upward slope.
To select the one or two-step advance of the drum a lever system is provided which comprises a stop lever provided with a stop or detent tooth which can be positioned in the return path of the support such as to interfere with an abutment surface thereof, and a control lever acting on the stop lever and having one end engageable with pegs arranged on the drum itself. The presence or absence of pegs determines the positions of the two levers, and consequently, the stopping or non-stopping of the support halfway through its return stroke, depending on whether the next advance movement is to be a one-step or two-step one.
However, in a device of this type, the start of the two-step advance movement is phase-advanced with respect to the one-step movement, thereby it cannot occur in phase therewith. This fact may result, on some articles, in pattern faults due to the phase-advanced variation of the peg arrangement with respect to that required for completing the knitting course being formed. To make that phase-advance less felt, one must retard as much as possible the start of the advance movement, i.e. increase the steepness angle of the first part of the cam upward portion, which controls the first phase of the two-step advance movement. However, this fact involves considerable accelerations in the drum and thus the necessity of providing powerful brakes to prevent the drum from being flung beyond the proper position. On the other hand, the provision of powerful brakes implies greater effort for the advance movement and, accordingly, greater wear of the components providing said advance movement and a frequent replacement of the brakes.
Another drawback of these conventional devices resides in the shocks produced during the return stroke of the support, when it strikes against the stop tooth of the stop lever, in normal operating conditions. In addition to generating noise, such shocks may damage the various mechanical components, and even affect the accuracy of the drum advance movement.
Moreover, the structure of the known devices is a comparatively complex and bulky one owing to the provision of two levers, respectively a stop lever and control lever. In addition thereto, the pawl support must be shaped such as to contact the stop lever, which increases its volume and mass.